Over the past years, a method has been perfected for perfusing an isolated rabbit kidney with whole blood using an artificial heart-lung system. During this time, the procedures and apparatus have been constantly modified so that whereas optimum function was obtained sporadically and for periods of 1 to 3 hours in the past, higher levels of renal function are now obtained in a greater percentage of experiments and for periods of 3 to 5 hours. AT this time, our kidney perfusion system appears to be one of the better functioning systems in operation today; however, with time there is a variable degree of deterioration in such functions as creatinine clearance, the concentrating ability and the PAH extraction ratio. These functional defects are more pronounced in all of the other kidney perfusion studies reported by others. It is proposed that efforts will be made to further improve and extend the function of the isolated kidney which is being used for collaborative studies with other nephrologists in this area to determine: a) whether a renal mechanism is responsible for the hypercalciuria observed in the phosphate depleted rabbit; b) the nature and site of action of a semi-purified natriuretic material obtained by extracting whole kidneys from volume expanded rats; c) the action of 25 hydroxy plus 1.25 dihydroxycholecalciferol on the isolated perfused kidney of normal, vitamin D deficient, and parathyroidectomized rabbits, and the factors involved in the conversion of 25 hydroxy to 1.25 dihydroxycholecalciferol; d) the effect of calcium on renin release in the isolated perfused kidney.